Apache Labs PowerSDR Update

Some experienced a crash in 3.3.7, especially when changing DSP Buffer Sizes or during RX/TX transitions. We believe this has been totally resolved.

SIGNIFICANTLY LOWER LATENCY

Receive latency is the time between when RF reaches your antenna and the corresponding audio is produced in your speaker or headphones. Similarly, transmit latency is, for example, the time between audio reaching your microphone and RF being on its way to your antenna. For many SDRs, especially those with sharp "brick wall" filters, the latency can be much larger than you might expect. Depending upon the radio design and various settings, SDR latencies can significantly exceed 100mS. Long latencies can create problems for the operator in contest operation, high-speed break-in CW, and even SSB rapid-turnaround VOX operation.

This release incorporates some technologies that allow us to achieve low latencies in the same category as leading conventional radios. Furthermore, we can do this with extremely sharp filters.

First of all, a couple basics:

* Sometime ago, we moved CW Transmit from software to the FPGA in the radio hardware. This means that CW transmit latency was already very low, really based upon your delay settings which are chosen to avoid any hot-switching of relays.

* It has always been the case that the Buffer Size setting on the Setup=>Audio/Primary tab effects latency. The lower the size, the lower the latency. However, the lower the size, the more CPU cycles are required. Depending upon the speed of your computer, you may be limited in how low you can go. Fortunately, this is not likely to have such a large impact on your latency. For a very rough estimate of the latency due to this buffer, divide the buffer size by the sample rate. For example, a buffer of size 256, at a sample rate of 192K, contributes only about 256/192000 = 1.33mS.

As of this release, there are some new features and corresponding controls to allow you to achieve much lower latency:

* Up until this release, "Filter Size" and "DSP Buffer Size" have been the same and there has only been one setting, called "DSP Buffer Size." Filter Size determines how sharp your filters are; higher filter size leads to sharper filters. However, higher DSP Buffer Size leads to more latency because we must collect enough samples to fill the buffer before the buffer can be processed. As of this release, DSP Buffer Size and Filter Size are separate and can be set by mode on the Setup=>DSP/Options tab. So, using a very low DSP Buffer size minimizes latency and using a high Filter Size leads to sharper filters. The trade-off here is that using lower DSP buffer sizes requires somewhat more CPU cycles and using a high Filter Size does as well. With a reasonably fast computer, you will likely be able to run at a DSP Buffer Size of 64, the minimum, except, perhaps, for the FM mode. With filter sizes of 1024 or 2048, the sharpness of our filters rival the best radios. However, larger sizes, up to 16384, are available if you need them.

* You now have a choice of Filter Type, with two types available: Linear Phase and Low Latency. In the past, our filters have always been Linear Phase. Linear Phase filters have the property that all frequencies are delayed by the same amount of time as the signal is processed through the filter. This means that the time-domain waveform of a signal that is totally within the passband will look the same at the input of the filter and the output of the filter. The Low Latency filter does not strictly comply with this same type of operation. With the Low Latency filter, signals at frequencies very near the lower and upper edges of the passband may experience more delay than signals at other frequencies. Comparing the two types of filters, beta testers have reported little, if any, difference in sound quality, no problems with several digital modes that have been tested, and no significant negative impacts at all from using the Low Latency filters. However, both filter types are provided for your comparison and your choice. Of course, the Low Latency filters provide lower latency. In fact, the latency of Linear Phase filters increases linearly with Filter Size while the latency of the Low Latency filters is very low and nearly independent of Filter Size.

Benchmark Comparisons:

* For CW/SSB receive, using minimum Buffer Sizes and Low Latency filters, our beta testers have measured receive latencies in the 15mS to 20mS range. Using minimum Buffer Sizes and Linear Phase filters, the latencies are 25mS to 30mS for a Filter Size of 1024 and 35mS to 40mS for a Filter Size of 2048. Using features such as noise blankers, EQ, and noise reduction will add some amount to that, depending upon the feature(s) and settings. These numbers compare with ~65mS and ~120mS using DSP Buffer sizes of 1024 and 2048, respectively, in prior software releases.

MINOR CHANGES The following list of values and states where added to the TX Profiles - selection for mic in or line in - 20dB mic boost - line in gain - CESSB state - PureSignal state

NOTE You WILL need to reset your database. This release will build a new wisdom file on first time use. Depending on your system, it may take a very long time to complete. Please be patient.